1. Field of the Invention
This application relates to polyamides and a process for their preparation. The polyamides are suitable for use as hot-melt adhesives and for the formation of shaped articles consisting completely or partly of such a polyamide. The invention also relates to a method for the manufacture of articles bonded with the present polyamides and to the articles so obtained.
2. Description of the Prior Art
Polyamides suitable for use as hot-melt adhesives are known. These polyamides must meet a number of requirements concerning adhesion, toughness, melting point, viscosity, elongation, elasticity, tensile strength, etc. To obtain favorable properties with respect to elongation and tensile strength highly viscous polyamides have been developed which are linear or almost completely linear. Such linear polyamines are obtained by reacting at least one bifunctional amine with molecular-distilled polymeric fatty acids consisting of 80-99% by weight of dimer with the amount of trimer and higher oligomers approximately compensated for by the amount of monomer present. Sometimes a lower dicarboxylic axid is also incorporated.
Polyamides with improved peel strength are obtained by incorporating not only ethylenediamine (EDA) but also another polyamine such as diethylenetriamine (DETA) and/or hexamethylenediamine (HMDA) in the reaction mixture. It has been found that DETA-based polyamides have particularly high peel strengths, however, probably due to the trifunctionality of DETA, polyamides based thereon are brittle and lack properties such as elongation. If, however, another diamine like e.g. HMDA is present, elongation is quite satisfactory but the peel strength is unsatisfactory when compared with DETA-based polyamides.
As pointed out polyamides exhibiting elongation are usually obtained from polymeric fatty acids having a high dimer content, typically exceeding 85% dimer. Also a certain relationship between the percentages of monomeric and trimeric constituents must be satisfied. Such pure dimers can only be obtained on a practical scale by molecular distillation of the dimer, which is cumbersome and adds considerably to the cost of the products obtained.
There is a need for polyamides combining toughness and elongation on the one hand with a high peel strength on the other hand. There is a particular need for non-brittle polyamides which are based on normal technical grades of polymeric fatty acids containing 75-85% of dimer acid.